The use of natural rubber in the medical and health care industries can expose atopic individuals to latex allergens. It is believed that as much as 6% of the general population, and as high as 12% of those working in the medical profession, are latex sensitive and display allergic reactions when exposed to proteins found in natural rubber (see M. McNulty, Rubber & Plastics News Jun. 25, 2001, page. 5). The symptoms of latex allergy range from mild contact dermatitis to life-threatening anaphylaxis, which includes a rapid drop in blood pressure and difficulty breathing. Even though a heightened awareness of latex allergy now exists, the number of people becoming sensitized to natural rubber is increasing as more professions require the use of latex gloves to avoid exposure to infectious agents. It is therefore not surprising, that with over 40,000 consumer products containing natural rubber (see Information from Allergy Advisor-Zing Solutions, http://allergyadvisor.com) an alternative protein free material is desired in many applications.
Although techniques exist for enzymatic deproteinization of natural rubber (see S. Kawahara, T. Kakubo, N. Nishiyama, Y. Tanaka, Y. Isono, J. T. Sakdapipanich, J. Appl. Polym. Sci. 78, 1510 (2000) and A. H. Eng, S. Kawahara, Y. J. Tanaka, Nat. Rubb. Res. 8, 109 (1993), as well as manufacturing practices that lower the total allergens present in latex goods, currently, the most effective way to provide protein free products is to use petrochemical derived synthetic rubbers. In fact, a report from the Johns Hopkins University School of Medicine recommended that the Food and Drug Administration mandate a switch from using stoppers made with natural rubber to using all synthetic rubber medicine stoppers (again see M. McNulty, Rubber & Plastics News Jun. 25, 2001). The synthetic rubber most closely related to natural rubber (NR), is high cis-polyisoprene. Typically, this material is prepared through the use of either stereospecific titanium catalysts or with alkyl-lithium initiators. Both of these systems are effective at providing protein free synthetic polyisoprene, however, the two polymers differ greatly with respect to their micro- and macrostructure. Polymerization of isoprene with titanium tetrachloride activated with a trialkylaluminum co-catalyst results in a material with upwards of 98% 1,4-cis content (see W. Cooper, in W. M. Saltman, ed., The Stereo Rubbers, John Wiley & Sons, New York, 1977, page 48). Polyisoprene produced commercially with alkyl lithium initiators generally does not have a cis content higher than 92%. Differences in microstructure, as well as macrostructure, allows the titanium polyisoprene (Ti-PI), but not the lithium polyisoprene (Li-PI), to display the unique advantage of strain-induced crystallization. It is the property of rapid crystallization that allows NR and Ti-PI to have high tensile strength and modulus even without the use of reinforcing fillers, a condition often found in gum stocks commonly used in the production of medical goods (see A. R. Bean, Jr., et. al., in H. F. Mark, N. G. Gaylord, N. M. Bikales, ed., Encyclopedia of Polymer Science and Technology, John Wiley & Sons, New York, Vol. 7, 1967, page. 823). Although Li-PI lacks the microstructural regularity of Ti-PI, it does have the advantage of being gel free with a narrow molecular weight distribution and linear macrostructure. These attributes allow Li-PI to display lower hysteretic properties at a lower cross-link density than NR or Ti-PI.
The use of Ti-PI and Li-PI in the medical and health care industries has gained acceptance, yet there is still need for a number of improvements. For example, the consistency of Ti-PI is very dependent on the aluminum to titanium ratio that is used during catalyst preparation (see W. Cooper, in W. M. Saltman, ed., The Stereo Rubbers, John Wiley & Sons, New York, 1977, page 48). If the ratio drops below unity the titanium is not sufficiently reduced, causing the formation of gel in the polymer. On the other hand, if the catalyst is over-reduced, oligomers will be produced giving the material a strong odor. Both of these unwanted side reactions must be carefully controlled. Current titanium systems also suffer from inferior activity and high levels of titanium must be used resulting in elevated levels of catalyst residues and terminating agents in the finished polymer. An overall reduction in foreign substances remaining in synthetic polyisoprene is of paramount importance when the production of a clean high performance material is desired.
Li-PI is considered a clean polymer due to the use of low levels of initiator during production and lack of extractables. However, the dependence of cis content on lithium concentration leads to a polymer with very high molecular weight (see H. L. Hsieh, R. P. Quirk, Anionic Polymerization Principles and Practical Applications, Marcel Dekker, Inc., New York, 1996, p. 201). The high molecular weight, coupled with a narrow molecular weight distribution, makes processing this material difficult. Commonly a low molecular weight fraction is added to commercial material to act as a processing aid.
It has now been envisioned that the use of neodymium catalyzed polyisoprene (Nd-PI), as a source of protein free synthetic natural rubber, may offer the combined advantages of both Ti-PI and Li-PI without their respective disadvantages. It is expected that Nd-PI with a cis 1,4-content as high as 98%, gel and oligomer free, linear with a moderate molecular weight distribution, easy to process, and low in volatile and extractable residues will be ideally suited for many clean high performance applications. A discussion of the synthesis, characterization, and compounded properties of this material is presented.